Measure the hot tub’s acrylic shell lip to lip, record length, width, corner radius, skirt, and strap details in inches, and order a cover that matches your manufacturer’s sizing rules.
A warped old cover can be an inch off in every direction, which is why it never seated right again after the first winter. Replacement covers are built to the spa, not to the cover you’re replacing, so measure the shell itself — across the widest outside points, in inches — before opening any order form.
Why You Measure The Tub And Not The Old Cover
The existing cover is the least reliable ruler in your backyard. Foam cores absorb water, vinyl shrinks in sun, and a cover that spent three seasons under snow can lose enough material to throw every dimension off by an inch or more.
What matters is the tub’s outermost acrylic or shell edge — the lip. Measure there, outside to outside, at the widest points, because many shells have softly rounded top edges that hide the true corner. For a square or rectangular spa, record length and width separately. Manufacturers commonly instruct adding about 1 inch total, or 1/2 inch per side, so the cover has room to sit and settle.
Most covers are ordered to a nominal size rather than your exact tape reading. If your spa is 84 by 84 inches, that’s a standard 84-inch cover — you supply the tub’s numbers and the maker maps them to their size chart.
The Measurements That Decide Whether It Actually Fits
Length and width are only half the order. Corner radius, skirt, and straps separate a cover that drops into place from one that gaps at the corners and flaps in the wind. Use a flexible tape in inches and take every reading at the widest point, not the trim line:
- Corner radius: lay a carpenter’s square into a corner and measure where the curve begins. Square tubs don’t need this; rounded or angled corners do.
- Skirt length: measure from the bottom of the cover down to the edge of the shell or trim — the flap that hangs over the cabinet.
- Strap placement: measure from the bottom of the skirt to the buckle or clip, and note where the straps sit along each side.
- Round tubs: measure straight across the center for the diameter.
- Hex or octagon tubs: measure one side plus the full span across opposite sides.
Write each number in inches as you go. Feet and rough guesses are how people end up with a cover a half inch short on two sides.
Measurements Worth Recording Before You Order
| Measurement | Where To Take It | Why It Matters |
|---|---|---|
| Length and width | Widest outside points of the shell, lip to lip | Sets the base cover size |
| Corner radius | Corner curve start, using a carpenter’s square | Prevents corner gaps on rounded tubs |
| Skirt length | Bottom of cover to shell or trim edge | Controls how the cover hangs on the cabinet |
| Strap length | Bottom of skirt to the buckle or clip | Keeps the cover secured in wind |
| Strap placement | Distance along each side to the strap | Must match your cabinet’s hardware |
| Diameter | Across the center of a round tub | Replaces length and width entirely |
| Side and span | One side plus opposite-side span, hex or octagon | Defines non-rectangular shapes |
Sizing runs slightly generous on purpose. A cover a touch large still seats and insulates, while an undersized one can fail to fit at all — so a little extra beats a tight squeeze. If your spa has an unusual shape or modified cabinet trim, compare your numbers against tested replacement covers before ordering, like the roundup of bath tub covers that fit standard and custom tubs. Canadian Spa Company’s own instructions for measuring a square or rectangular hot tub confirm the same outside-to-outside method.
Mistakes That Send Covers Back
Almost every return traces to one of five preventable errors. Most common: measuring the old cover instead of the shell. Next: measuring to an inside edge or trim line rather than the outer acrylic lip. On rounded or angled tubs, skipping the corner radius guarantees gaps. Ignoring skirt length and strap placement produces a cover that fits the top but won’t stay put. And using feet or eyeballing throws off the one dimension that decides everything.
Acrylic shells also round off near the top, so a measurement taken a half inch in from the edge reads short — measure at the widest point every time. Ordering directly to your tape numbers without checking the maker’s size chart is the last trap; nominal sizes rarely match your reading exactly.
FAQs
Should the cover be exactly the same size as the tub?
Not exactly. Most manufacturers suggest a cover roughly 1 inch larger overall, or about 1/2 inch extra per side, so it seats properly and accommodates foam settling. A slightly oversized cover performs far better than an undersized one, which may not seat correctly and can lose insulation performance.
Can I measure my old cover if I never measured the tub?
Use it only as a rough cross-check, never as the primary source. Vinyl shrinks in sunlight and foam cores soak up water, so an older cover may read an inch or more off in any direction. Always measure the spa’s acrylic shell at its widest outside points and order from those numbers.
How do I measure a round or octagonal hot tub?
For a round spa, measure straight across the center for the diameter. For a hexagonal or octagonal tub, record the length of one side plus the overall span across opposite sides. Both shapes also need skirt length and strap placement measured the same way as a square tub.
References & Sources
- Canadian Spa Company. “How to Measure a Square or Rectangle Hot Tub for a Replacement Hard-Top Cover.” Supports outside-to-outside shell measurement and per-side sizing allowance.
